In a power trowel of the above described character, the entire weight of the machine is supported by the rotating blades, which are always engaged with the surface to be finished. A strong torque is therefore imposed upon the chassis that tends to rotate it in the direction opposite to that of blade rotation. The operator resists such torque by maintaining his grip upon the handle and controls the movements of the machine by applying appropriate vertical forces to the handle. With power trowels heretofore available, the connection between the handle and the chassis was a rigid one, and rather strong alternate upward and downward forces had to be applied to the handle during the course of a finishing operation. Upward force on the handle tended to tilt the chassis forward, causing the blade tips to bear most heavily against the surface at the fronts of their orbits so that the machine moved oppositely to the direction of the blade tips at the front of the orbit. Downward force on the handle caused the machine to move in the opposite sideward direction. With a moderate upward force on the handle the machine could be kept stationary.
The necessity for exerting rather large upward and downward forces on the handle was in itself fatiguing to the operator. In addition special difficulty was encountered in keeping the machine at a standstill, owing to the difficulty in maintaining the right amount of upward force while resisting the relatively high torque force on the handle.
These difficulties were aggravated by the substantial vibration that normally attends operation of a power trowel and results from irregularities in the freshly laid concrete surface that the blades engage. Such vibration causes abrupt up and down movements of the rear end of the handle relative to the operator's hands and thus imposes unintended vertical forces upon the handle that interfere with control of the machine.
Bearing in mind that a power trowel is a heavy and powerful machine, it will be apparent that, apart from the matter of operator fatigue, important safety considerations are involved in the ability to maintain positive control over its direction and speed of movement, and particularly in the ability to stop its movement accurately and to assuredly keep it stopped.
Heretofore these difficulties and disadvantages in the control of power trowels seem to have been accepted as inevitable, and apparently they were not recognized as undesirable. Accordingly, it has not been obvious that improvements could be made in the facility and accuracy with which a power trowel can be controlled, much less has it been obvious how to achieve such improvements.